Among the new compounds which can be prepared by the process of the invention, (+) trans-6,10-dimethyl-bicyclo[4.4.0]dec-1-en-3-one is a naturally occurring compound. The said ketone was in fact isolated for the first time from vetiver Reunion oil (Vetiveria zizanioides), by means of an extremely complex and expensive process. The said isolation process first consisted in separating the ketonic fraction of the natural essential oil by treating this letter with the Girard's reagent [see Helv. Chim. Acta 22, 640 (1939)]. After hydrolysis the recovered pure ketonic fraction was the submitted to several fractional distillations, a new ketonic fraction containing a high proportion of the desired compound was thus obtained. Pure (+) trans-6,10-dimethyl-bicyclo[4.4.0]dec-1-en-3-one was finally obtained by preparative vapour phase chromatography. It has to be pointed out that said ketone is a very minor constituent of natural vetiver oil, this latter containing in fact a proportion from about 0.1-0.2% (parts by weight) of said compound.
The pure ketone possesses a very distinct and powerful odour presenting an original woody character. Moreover, this odour is very stable and easily reproducible whereas the odour of the natural essential oil, which results from the overlapping of the odour of each individual constituent of the said oil, can vary depending on the origin and the purity of the said natural essential oil.
Owing to its particular olfactive properties the above mentioned bicyclic ketone can be widely used in the art of perfumery. Its use is broader than that of vetiver oil itself and it enables the perfumer to create totally original woody notes. Furthermore, the said ketone possesses a very appreciated reinforcing and fixative effect.
Hitherto, (+) trans-6,10-dimethyl-bicyclo[4.4.0]dec-1-en-3-one represents the first known example of a compound having a ketonic structure with a skeleton containing 12 carbon atoms found in vetiver oil. The said discovery has thus encouraged the man in the art to further explore a whole series of analogous bicyclic derivatives which represent a new class of valuable perfuming ingredients.
The above ketone can also be obtained by a process consisting in the separation of the two enantiomeric constituents of the known racemic compound, e.g. via the corresponding semi-carbazone derivative [see J. Am. Chem. Soc. 80, 6551 (1958)]. The said process can also be effected by separating the corresponding racemic unsaturated alcohol via the ester prepared from .alpha.-bromo-campher sulfonic acid the said bicyclic alcohol [see J. Am. Chem. Soc. 91, 535 (1969)], and followed by a regeneration and subsequent oxidation of the thus obtained optically active alcohol.
By means of a novel process of synthesis industrially and economically more advantageous than the isolation of the said ketone from the natural vetiver oil, it is now possible to put to the disposal of perfumers and flavourists a practically pure trans-6,10-dimethyl-bicyclo[4.4.0]dec-1-en-3-one. The difficulties of supply, storage and purification of the natural essential oil are thus avoided.